The invention relates to the extrusion of articles and, more particularly, to a vacuum sizing device including a sizing die used to size and shape plastic extrusions pulled through the sizing die disposed in an evacuated chamber.
In the extrusion of plastic articles, such as cylindrical rods used as hinge rods in constructing modular plastic conveyor belts, it is common to size and shape the extruded rod with a series of sizing dies aligned along the length of a chamber. An extruder nozzle injects hot plastic into the chamber through an entrance opening at one end. The plastic is extruded through the entrance opening and the aligned dies and out of the chamber through a similarly sized exit opening. Typically, the chamber includes a liquid, such as water, that surrounds the extrusion and serves to cool the extrusion and lubricate the dies. The head of water and the pressure exerted on the extrusion can be adjusted by evacuating the top of the chamber.
For cylindrical rods, each sizing die has a closed circular opening aligned with the circular openings of the other of the series of dies. The dimensions of the openings are roughly the same as that of the desired cross-section of the extruded rod. The series of dies is typically mounted in a vacuum chamber, which causes the hot, moldable plastic to expand outwardly as it cools to take the shape of the die openings through which it is pulled.
One difficulty with extruding rods is getting the extrusion process started. Especially tedious is the job of threading the plastic through the series of closed circular die openings. One method is to backfeed a previously molded, thin, cold rod section through the exit opening of the chamber and through the die openings to the entrance opening. The soft, hot plastic exiting the extruder nozzle that adheres to the end of the cold rod is manually strung through the openings until it can be engaged by the puller. During this procedure, the extruder nozzle is adjusted to eject plastic at a low rate. Once the plastic is threaded through the chamber, the rate can be increased. Frequently, however, the bond between the end of the cold rod and the hot plastic breaks before the threading process is completed. The resulting mess must be cleaned up, and the process repeated until threading is complete.
To solve the threading problem, the dies of some vacuum sizing devices are split into halves that can be separated. In conjunction with a chamber that can be opened for access to the separated die halves, the hot plastic can be pulled from the opening and directed into the separated die halves through the separation. Once the plastic material extends through the series of dies, the die halves are closed, and the process can proceed. Although split sizing dies solve many of the threading difficulties, they introduce other problems.
Because split dies are split through the circular opening, unless the die halves mate exactly without offset and are fully flush, the exposed corners around the split are highly susceptible to wear. Another problem with split die halves is that a seam is extruded on one or both sides of the rod. In many applications, such a seam is unacceptable. For example, a seam on a pivot rod for a modular plastic conveyor belt, besides being cosmetically deficient, can interfere with good belt articulation, cause premature wear, and make the rod generally difficult to insert into position during belt construction.
Consequently, there is a need for an easy-to-load vacuum sizing device that can be used in the extrusion of seamless plastic rods.